A phased array antenna such as a Massive-MIMO antenna expected to be used for 5G has a plurality of antenna elements, and is capable of controlling radiation direction and beam shape of the electromagnetic wave to be outputted by controlling the amplitude and phase of each antenna element.
In order to radiate electromagnetic waves of a desired radiation direction and beam shape from the phased array antenna as described above, it is desirable that the transmission properties of the plurality of antenna elements are equal to each other. However, in general, the transmission properties differ from one element to another due to individual differences of the antenna elements and the like. Therefore, it is important to calibrate the antenna elements so that the amplitude and phase take a predetermined value (for example, equal phase and equal amplitude) for each antenna element, before the use of the phased array antenna.
Conventionally, as a calibration method, an element electric field vector rotation method (REV method) for determining an amplitude and a phase is known (for example, see Non-Patent Document 1), in which the amplitude and the phase of each antenna element are determined based on the measurement result of the amplitude of the electric field generated from the array antenna in the far field, while rotating the phase of each antenna element of the array antenna.
Here, the far field is defined as a position spaced apart from the antenna by a distance R, that satisfies the following formula (1), or longer, with respect to the maximum diameter D (aperture size) of the antenna, where λ is the free space wavelength.R>2D2/λ  (1)
In general, as shown in FIG. 23, of the electromagnetic field radiated from the antenna aperture plane, the area close to the antenna aperture is called a reactive near field area (extreme near field) mainly constituted of electromagnetic field components that do not contribute to radiation, and the area where the directivity does not change according to the distance from the antenna aperture is called a radiating far field area (far field). In general, the directivity of the antenna refers to the directivity measured in the radiating far field area.
The radiating near field area (near field), which is the area between the reactive near field area and the radiation far field area, is an area whose directivity changes according to the distance. As a method of measuring the properties of an antenna with strong directivity like the Massive-MIMO antenna, a near field measurement method is known in which electromagnetic fields are measured in the radiation near field area and the directivity in the far field is obtained through a calculation.
Now, the REV method is an effective method that can determine the relative phase of each antenna element only by measuring the amplitude, but since it is necessary to perform measurement for each antenna element, there is a problem that calibration time increases as the number of antenna elements increases. In particular, in a massive-MIMO antenna using a multi-element array antenna, there is a possibility that the measurement time increases enormously.
As a means to solve the problem of the REV method mentioned above, a method is proposed in which the aperture plane distribution of the antenna at the distance R=0 is estimated by back projecting the near field measurement result, so that the relative phase of each antenna element of the array antenna is measured at once (See, for example, Patent Document 1).